The present invention relates to a high-power semiconductor laser having a current noninjected region in the vicinity of an end face (facet). In particular, the invention enhances a facet Catastrophic Optical Damage level higher and provides high reliability in long-term continuous operations.
A semiconductor laser device is utilized in various fields such as an excitation light source for a light amplifier used in the field of communications. These lasers are required for high power operations. However, there is a problem that it is difficult to obtain the semiconductor laser having enough life for carrying out the high power operations. In general, facet COD (Catastrophic Optical Damage) is known as the main factor of deterioration of the semiconductor laser. The COD is caused by the following steps. First, the non-radiative recombination is caused by defects produced in the vicinity of the facet. This results in an increase of the temperature. In addition, due to the temperature increase, the width of band gap is decreased and light absorption occurs, resulting in a vicious cycle of further temperature rise due to this. These steps induce melting of the facet and light output is deteriorated. Finally, a non-reversible destruction occurs
According to the above description, it becomes important to reinforce the facet for obtaining the high-power semiconductor laser, in terms of preventing the facet COD. As one means for preventing the facet COD, there is a method in which no current is injected in the vicinity of the facet. In this case, since a current injection is suppressed in the vicinity of the facet, the vicinity of the facet becomes non-excitation state. Because of this, the non-radiative recombination is suppressed, which enables to improve the facet COD level. As specified examples, there are following methods. That is, no current is injected in the facet by forming an insulating film such as SiN under an electrode formed on the facet, no current is injected in the facet by forming a current blocking layer comprising a semiconductor layer on the facet, and no current is injected in the facet by carrying out ion implantation in the vicinity of the facet.
All the above methods have complicated fabrication steps. Further, the method causes damage to the semiconductor laser device, in which no current is injected in the facet by carrying out ion implantation in the vicinity of the facet. Furthermore, all these facet current blocking structures are formed at a distance from a waveguide layer. When providing the current blocking structure, it can be considered that a current wraparound will occur. The current wraparound is considered to cause serious influence as the distance between the current blocking structure and the active layer increases. In the case where the influence of the current wraparound becomes serious even if the current blocking structure is formed on the facet, current is wrapped around the facet, resulting in function degradation of the facet current blocking structure. Consequently, in consideration of the current wraparound, it is necessary to form wide facet current blocking region. When the facet current blocking region is widely formed, an influence caused by light absorption becomes serious in the region, so that the properties (threshold current, slope efficiency, temperature characteristic, or the like) of the semiconductor laser device are deteriorated. Because of this, any conventional methods described above are not necessarily preferable when forming a current blocking structure in the vicinity of the facet.
The invention is aimed at solving the above problems. An object of the invention is to provide a semiconductor laser device having high facet COD level, and high reliability in long-term continuous operations, by providing a current blocking structure in the vicinity of the facet. According to the structure, fabrication processes become easy, no damage are caused to the semiconductor laser device, and the property deterioration can be minimized.
To achieve the above object, the invention relates to a semiconductor laser device comprising:
an active layer;
an n-type waveguide layer;
a p-type waveguide layer,
the active layer being interposed between the n-type and p-type waveguide layers;
n-type and p-type cladding layers formed so that outsides of the n-type and p-type waveguide layers are interposed therebetween;
first current blocking layers formed to define a stripe-shaped current injected region extending in a direction where a front facet of the device from which a laser light is emitted and a rear facet of the device opposing thereto are connected; and
a second current blocking layer formed to transverse the stripe-shaped current injected region in a vicinity of the front facet,
wherein the first current blocking layers and the second current blocking layer are made of the same layer.
According to the semiconductor laser device thus structured, the first current blocking layers, between which the stripe-shaped current injected region extending in the resonator direction is interposed, and the second current blocking layer, which is formed in order that no current is injected in one side or both sides of the vicinity of the facet, are made of the same layer, namely they are of the same composition and of the same film thickness. Because of this, fabrication processes become easy without increasing the number of processing steps compared with the conventional method. Consequently, the current blocking structure can be provided in the vicinity of the facet without damaging the semiconductor laser device. Accordingly, it is possible to provide the semiconductor laser device having high facet COD level and high reliability in long-term continuous operations. Not only the front facet but also the rear facet may be provided with the current blocking layer. One of both side portions of the second current blocking layer reaches the facet.
In the invention, a refractive index waveguide structure can be formed by an equivalent refractive index difference between the current injected region and the region of the current blocking layers. Further, a carrier blocking layer having larger energy gap than that of the waveguide layer is provided between the active layer and the waveguide layers, thereby a carrier is confined and a waveguide mode in the epitaxial direction can be expanded. Consequently, the facet COD level can be further improved by suppressing light intensity concentrated on the facet active layer.
In the invention, it is preferable that the first and second current blocking layers are formed inside the waveguide layer. The first and second current blocking layers may be formed in adjacent to the waveguide layer. In this case, when the width of the second current blocking layer is too large, waveguide loss is increased. Because of this, the second current blocking layer is desired to have a width within the range of 2 to 25 xcexcm in practical use.
In the structure in which the current blocking layer in the vicinity of the facet having lower refractive index than that of the waveguide layer is formed in the waveguide layers or in adjacent to the waveguide layers, the waveguide mode profile in the vicinity of the facet can be shifted from the active layer by the use of the low refractive index layer in the vicinity of the facet. Accordingly, the beam energy density near the active layer in the vicinity of the facet can be reduced, and it is possible to provide the semiconductor laser device having substantially improved facet COD level and high reliability in long-term continuous operations. Further, since the waveguide layers are formed in the vicinity of the active layer, influence caused by the current wraparound into the active layer can be reduced by providing the facet current blocking structure in the waveguide layers. By doing this, the current blocking region necessary for ensuring improvement of the facet COD level and high reliability in long-term continuous operations, can be narrower compared with the case where the current blocking structure is provided over the waveguide layers. When the current blocking region becomes narrow, influence due to light absorption can be decreased, resulting in reducing degradation of the properties (threshold current, slope efficiency, temperature characteristic, or the like) of the semiconductor laser device.
Further, in the invention, it is preferable that the active layer comprises InGaAs and the waveguide layers are made of GaAs not including Al.
It becomes possible to use GaAs not including Al for forming a waveguide layer by the use of InGaAs for forming quantum wells of the active layer. Because a regrowing interface is not subject to oxidation during the formation processes of the current blocking layer, stability of processes can be achieved so that fine film can be formed. The reduction of electrical resistance and thermal resistance can be accomplished by using a waveguide layer made of GaAs not including Al.
When the current blocking layer in the vicinity of the facet has lower refractive index than the waveguide layer, a waveguide mode in the vicinity of the facet is influenced by the layer. Namely, the waveguide mode can be controlled by changing position, distance from the face and refractive index of the current blocking layer in the vicinity of the facet imbedded in the waveguide layer.
It is generally known as means for improving the facet COD level to decrease a beam energy density near the active layer formed on the facet. The beam energy density near the active layer is represented by a light confinement factor xcex93. By providing a current noninjected region, position, width and refractive index of the current blocking layer in the vicinity of the facet are designed to satisfy the relationship between a light confinement factor xcex931d Injection at the current injected region obtained through one dimensional slab waveguide path in the film thickness direction and a light confinement factor xcex931d Non-injection in the film thickness direction at the current noninjected region in the vicinity of the facet as follows:
xcex931d Injection greater than xcex931d Non-injection
By doing this, it is found that beam energy density near the active layer on the facet can be reduced and the facet COD level can be raised. For example, in the embodiment mentioned below, the light confinement factor xcex931d Injection at the current injected region shows 0.0084, whereas the light confinement factor xcex931d Non-injection in the film thickness direction at the current noninjected region in the vicinity of the facet shows 0.0071. Consequently, the beam energy density near the active layer in the vicinity of the facet, can be reduced.
The light confinement factor xcex93 of one dimensional slab waveguide path used herein is defined as the following formula.                               Γ                      1            ⁢            d                          =                                            ∫              b              a                        ⁢                                                            "LeftBracketingBar"                                      E                    ⁡                                          (                      x                      )                                                        "RightBracketingBar"                                2                            ⁢                              ⅆ                x                                                                        ∫              B              A                        ⁢                                                            "LeftBracketingBar"                                      E                    ⁡                                          (                      x                      )                                                        "RightBracketingBar"                                2                            ⁢                              ⅆ                x                                                                        [                  Formula          ⁢                      xe2x80x83                    ⁢          1                ]            
wherein E(x) is an electric field in the film thickness direction, A and B are a maximum value and a minimum value of electric field coordinates, respectively. Further, a and b are values determined by an interface of the active layer.
Actually, in the waveguide mode, beams propagate dynamically. The state in which the waveguide mode is influenced by the current blocking layer in the vicinity of the facet, can be analyzed by e.g. a computer simulation using the beam propagation method. The position, width, and refractive index of the current blocking layer in the vicinity of the facet, which is imbedded in the waveguide layer, are designed to satisfy the relationship between a light confinement factor xcex932d injection at the current injected region and a light intensity rate xcex93hu 2d Facets at the active layer in the front facet with respect to a propagation mode when propagating the waveguide mode at a current injected region into a current noninjected region by the beam propagation method as follows:
xcex932d Injection greater than xcex932d Facet
By doing this, it is found that the waveguide mode is controlled and the facet COD level can be increased. Further, beam energy density near the active layer on the facet is designed to take on values between adjacent inflection points including a minimum value, thereby the semiconductor laser device having a higher facet COD level can be obtained. For example, FIG. 7 is a graph of an analysis obtained by a computer simulation (BPM CAD produced by Optiwave Corporation) using the beam propagation method. In the structure shown in FIG. 3 according to embodiments mentioned below, an analysis is carried out on the relationship between a light intensity rate (light confinement factor) xcex93 of the active layer with respect to the propagation mode at the side portion (reference numeral 23a in FIG. 3C) opposed to the side portion adjacent to a front facet 40, which is one of both side portions of the current blocking layer 23 provided in the front facet, and the width W of the current blocking layer 23 (current noninjected region). The light confinement factor xcex93 used herein is defined as the following formula.                               Γ                      2            ⁢                          xe2x80x83                        ⁢            d                          =                                            ∫                              c                ,                d                                            a                ,                b                                      ⁢                                                            "LeftBracketingBar"                                      E                    ⁡                                          (                                              x                        ,                        y                                            )                                                        "RightBracketingBar"                                2                            ⁢                              ⅆ                x                            ⁢                              ⅆ                y                                                                        ∫                              C                ,                D                                            A                ,                B                                      ⁢                                                            "LeftBracketingBar"                                      E                    ⁡                                          (                                              x                        ,                        y                                            )                                                        "RightBracketingBar"                                2                            ⁢                              ⅆ                x                            ⁢                              ⅆ                y                                                                        [                  Formula          ⁢                      xe2x80x83                    ⁢          2                ]            
wherein E(x, y) is an electric field, (A, B) and (C, D) are a maximum value and a minimum value of electric field coordinates, respectively, and (a, b) and (c, d) are values determined by an interface of the active layer.
As apparent from FIG. 7, in this case, the light confinement factor becomes a minimum value when the current blocking layer on the facet has a width of about 5 xcexcm or 15 xcexcm. It should be noted that the optimum value of the width of the current blocking layer in the vicinity of the facet is designed as necessary depending on the layer structure in order to minimize the light confinement factor at facets. Further, the width of the current blocking layer, which minimizes the light confinement factor, is varied by changing position, width and refractive index of the current blocking layer in the vicinity of the facet.